use std::borrow::Cow;

use axum::http::Uri;
use axum::http::{HeaderMap, Method, StatusCode, header};
use axum::response::{IntoResponse, Response};

const DEV_HINT: &str = r#"<!doctype html>
<html lang="en"><head><meta charset="utf-8"><title>filebrowser-ng</title></head>
<body style="font-family:system-ui;display:grid;place-items:center;min-height:100vh;margin:0;background:#f6f7f9">
<div style="max-width:520px;padding:24px;border:1px solid #e5e7eb;border-radius:12px;background:#fff">
<h1>filebrowser-ng</h1>
<p>The frontend has not been built yet.</p>
<p>In development, run <code>just dev-web</code> in a second terminal and open
<code>http://localhost:8080</code> (Trunk serves the app and proxies <code>/api</code> to this backend).</p>
<p>If you hit this page directly from the backend, make sure Trunk's output
directory is visible — the server looks in <code>target/web</code> or <code>$FBNG_DIST</code>.</p>
<p>For the production single binary: <code>just build</code>.</p>
</div></body></html>"#;

/// Serve the frontend: static assets, with a fallback to index.html so that
/// client-side routes (e.g. `/s/<token>`) work.
pub(super) async fn fallback(method: Method, uri: Uri, headers: HeaderMap) -> Response {
    let path = uri.path();
    if path.starts_with("/api/") {
        return (StatusCode::NOT_FOUND, "unknown endpoint").into_response();
    }
    if method != Method::GET && method != Method::HEAD {
        return StatusCode::METHOD_NOT_ALLOWED.into_response();
    }

    let rel = path.trim_start_matches('/');
    let asset = if rel.is_empty() {
        crate::assets::get_asset("index.html")
    } else {
        crate::assets::get_asset(rel)
    };

    match asset {
        Some((bytes, mime, cache, etag)) => {
            // The embedded assets carry a content hash, so a repeat visit can
            // be answered without sending the body again.
            if let Some(tag) = &etag
                && headers
                    .get(header::IF_NONE_MATCH)
                    .and_then(|v| v.to_str().ok())
                    .is_some_and(|v| v.split(',').any(|t| t.trim() == tag || t.trim() == "*"))
            {
                return (
                    StatusCode::NOT_MODIFIED,
                    [(header::CACHE_CONTROL, cache), (header::ETAG, tag.clone())],
                )
                    .into_response();
            }
            let mut res = (
                [(header::CONTENT_TYPE, mime), (header::CACHE_CONTROL, cache)],
                body(bytes),
            )
                .into_response();
            if let Some(tag) = etag
                && let Ok(v) = tag.parse()
            {
                res.headers_mut().insert(header::ETAG, v);
            }
            res
        }
        None => match crate::assets::get_asset("index.html") {
            Some((bytes, mime, _, _)) => (
                [
                    (header::CONTENT_TYPE, mime),
                    (header::CACHE_CONTROL, "no-cache".to_string()),
                ],
                body(bytes),
            )
                .into_response(),
            None => DEV_HINT.into_response(),
        },
    }
}

/// Asset bytes as a response body, without copying the embedded ones.
fn body(bytes: Cow<'static, [u8]>) -> axum::body::Bytes {
    match bytes {
        Cow::Borrowed(b) => axum::body::Bytes::from_static(b),
        Cow::Owned(v) => axum::body::Bytes::from(v),
    }
}